1. Field of the Invention
The present invention relates generally to a system for testing properties of a lens, and more specifically to a system for testing lenses that are used in scanners or the like. In particular, the system determines a lens' through-focus modulation transfer function, distortion, focal length, and color registration, and then outputs these values to a user.
2. Description of the Related Art
Scanners typically employ a lens to focus light from an image (e.g., a document image, a graphics image, etc.) onto a detector, such as a charge coupled device (hereinafter "CCD"). The detector, in turn, converts light from the document image into electrical signals, from which data corresponding to the document image is generated. As might be expected, the accuracy of this data is based on the quality of the lens used to focus the light from the document image. In particular, the accuracy of the data is dependent, at least in part, upon the modulation transfer function (hereinafter "MTF") of the lens.
The MTF of a lens defines how sharply the lens is able to focus. That is, for a line pattern having a particular frequency, such as 35 line-pairs-per-millimeter (hereinafter "lp/mm"), the MTF corresponds to the contrast of an image formed by the lens relative to a contrast of an original image. A high MTF value is, of course, preferable, since it means that the lens will have high contrast and, therefore, will provide more accurate document imaging. Accordingly, systems have been devised to test a lens' MTF values at discrete spatial frequencies. Specifically, such systems focus light from a target having a known line pattern of a particular frequency in lp/mm, through the lens, and onto a CCD. Thereafter, the MTF of the lens at the frequency of the target line pair pattern is determined mathematically based on the light received from the lens. One problem with this method is that in order to determine the lens MTF at a plurality of frequencies, a plurality of line patterns, each having a lp/mm frequency of a desired MTF frequency to be measured, is required to be placed on the target.
Other problems also exist with conventional lens testing systems because those systems are not able to obtain a through-focus MTF for the lens, and do not take into account the MTF of the CCD, among other things, when determining the lens' MTF. That is, the MTF of the CCD affects the image data generated from the light incident on the CCD. Thus, conventional lens testing systems, which do not account for the MTF of the CCD, are unable to determine a lens MTF accurately.
Another problem which exists with conventional lens testing systems is that a lens may not be tested over its full field of view. Systems which test a lens in the center of the field of view are known and it is known to translate a test apparatus from the center of the field of view to the extreme edges of the field of view and to repeat the test at each location of the field of view for which the lens performance is to be tested. It is desirable, especially in scanning systems to determine the lens performance characteristics e.g. MTF, distortion and color registration as well as other lens characteristics at a plurality of field positions along the lens field of view and to reduce the time and expense of performing a separate lens test at each of the plurality of field positions along the lens field of view for which test data is required.
Moreover, conventional lens testing systems do not test other properties of a lens which can affect light transmission and thus document imaging. Examples of these other properties include color registration, focal length, and distortion each of which may vary along the field of view of the lens and through the lens depth of field. Distortion refers to the amount that an image is distorted when light is transmitted through the lens. As might be expected, the less distortion that a lens produces, the better. Color registration refers to differences in light transmission for different wavelengths (i.e., colors) of light. That is, a lens may transmit different colored light to different locations, e.g. each color light may have a unique focal plane, thereby leading to registration errors between different colors of the same image. These registration errors can significantly affect the way in which a color image is perceived. Last, but not least, a lens' focal length affects its ability to transmit light accurately.
For at least the foregoing reasons, conventional lens testing systems are inadequate, particularly with respect to testing lenses for use with document scanners or other types of imaging systems. Accordingly, there exists a need for a lens testing system which addresses the foregoing drawbacks of conventional lens testing systems. In particular, there exists a need for a lens testing system which is able to provide an accurate, through-focus measurement of a lens MTF. In addition there exists a need for a lens testing system which is able to quickly provide a measure of a lens MTF, and other lens properties, such as distortion, color registration, and focal length at several field locations.